The present invention relates generally to road safety devices and, more particularly, to rumble strips and methods and apparatus for forming same.
Depressions, commonly referred to as rumble strips or sonic nap alert patterns (SNAPS), are formed in the surfaces of roads to alert drivers when their vehicles have drifted out of the travel lane of the road or when the vehicle is approaching a dangerous portion of road, such as a stop sign or toll booth. Rumble strip depressions are typically either uniformly spaced apart or are placed in spaced apart groupings in what is known as a skip pattern. Although there is no uniform standard for rumble strips, many highway departments require a five inch separation between adjacent depressions, with each depression having dimensions of seven inches by sixteen inches and a minimum depth of one-half inch. The long dimension of each depression normally extends perpendicular to the longitudinal axis of the road.
Rumble strip depressions are conventionally formed in various fashions, including by using a roller drum to impress the depressions into asphalt roads, using forms to shape wet concrete, or milling or cold planing the depressions in either asphalt or concrete roads. Using roller drums to form the rumble strips is desirable because the depressions are quickly and easily formed by simply rolling the drum over the road surface. Roller drums, however, can only be used on asphaltic roads, and the impressed depressions may tend to rebound over a period of time, thereby reducing the effectiveness of the rumble strips. Using forms to shape the depressions is of limited applicability because the forms can only be used with wet concrete. Milling of rumble strips is more versatile because the depressions can be formed in asphalt as well as hardened concrete.
Various types of milling machines are currently used to cut rumble strip depressions. These machines typically utilize a cylindrical cutting head that is formed by ganging together a plurality of circular diamond saw blades or by attaching tungsten-carbide-cutting elements to a drum. The cutting head is rotated about an axis parallel to the road surface and is repeatedly raised and lowered to cut spaced apart rumble strip depressions as the milling machine travels down the road in a continuous or step-wise fashion. The resulting depressions are typically rectangular in configuration, with curved forward and rearward walls and vertically extending lateral walls. The distance between the forward and rearward walls is variable and is determined by the length of time that the cutting head remains in contact with the road surface. The spacing between the lateral walls is fixed and corresponds to the axial length of the cutting head.
The vertically extending lateral walls in conventional rumble strip depressions can cause several problems. First, when a vehicle drifts off the travel lanes and onto a rumble strip positioned on the road shoulder, the vertical lateral walls cause the tires to suddenly drop laterally from the road surface into the rumble strip depression. This abrupt movement may pull the vehicle even further onto the shoulder and may momentarily distract the vehicle driver. Once the vehicle tires are within the rumble strip depressions, the vertical lateral walls may cause the driver to overcorrect in an attempt to remove the tires from within the rumble strip depression. These problems may be particularly serious for smaller cars, motorcycles and bicycles that are lighter and have smaller tires.
Another drawback of rumble strip depressions having vertical walls is the sharp 90xc2x0 corner formed between the bottom of the depression and the vertical walls creates an area of localized stress that can cause cracking of the pavement. This cracking may accelerate degradation of the road and require costly repairs or replacement of the road. Water and other debris may also accumulate in those 90xc2x0 corners and further contribute to road degradation.
As a result of the foregoing problems, a need has developed for a rumble strip depression that is less likely to interfere with vehicle steering and which does not cause premature failure of the road surface.
In one aspect, the present invention is directed to a cutting head for milling rumble strip depressions in an upper surface of a road. The cutting head includes a drum having opposed ends and a center rotational axis extending between the opposed ends. A plurality of milling elements are mounted on an outer surface of the drum and have cutting surfaces positioned to define a plane curve as the drum is rotated about its center rotational axis. The plane curve has a transverse diameter that varies in dimension along said center rotational axis so that the plane curve approximates at least a portion of an elliptical or other curved configuration. Positioning the cutting surfaces in the plane curve allows the cutting head to be used to mill rumble strip depressions that have a concave bottom that curves upwardly along two perpendicular axes. The milling elements preferably are milling teeth that are removably positioned within holders 24 welded or otherwise secured to the outer surface of the drum. The milling teeth also preferably having cutting tips formed of a material than includes tungsten carbide. The cutting tips are positioned so that they cut in the plane curve by either configuring the drum in the shape of an ellipsoid and using the same sized milling elements across the surface of the drum or by providing a cylindrical shape to the drum and using different sized milling elements to achieve the desired profile. In an alternate embodiment, the drum and milling elements are formed by stacked saw blades with interposed spacers.
In another aspect, the present invention is directed to the resulting rumble strip depressions. In the preferred embodiment, the depressions have only two spaced apart sides and the curved bottom curves upwardly along the first axis to form the two sides. In another embodiment, the bottom curves upwardly along the second axis and connects to two lateral and spaced apart side walls.
The invention also includes a method of milling the rumble strips using the cutting head described above and a milling machine that can otherwise be of a conventional construction.
The rumble strip depressions of the present invention have bottoms that curve along both perpendicular axes, in contrast to conventional rumble strips that curve upwardly along only one axis. The resulting forward and rearward side walls curve in a horizontal plane and, when painted with traffic paint or another light reflective coating, is more readily observable because light from vehicle headlights is reflected in multiple directions. The curved nature of the bottoms of the depressions also reduces the areas of localized stress that can lead to cracking and degradation of the road surface. Vehicle-induced wind currents more easily remove debris entering the rumble strip depressions because there are no sharp corners within the depressions where the debris can be shielded from the wind currents. The rumble strip depressions can also be easier to mill because less road material is removed in the formation of the rumble strip depressions of the present invention.